A Highly Enantioselective and Diastereoselective Synthesis of Cyclobutanes via Boronic Esters

[formula: see text] Deprotonation of enantiopure (R,R)-1,2-dicyclohexyl-1,2-ethanediol 1-chloro-4-cyanobutylboronates 5 with LDA followed by treatment with anhydrous magnesium bromide yields (R)-(trans-2-cyanocyclobutyl)boronic esters 7 in high diastereomeric and enantiomeric purity. No cyclobutane formation has been observed in the absence of at least a catalytic amount of magnesium halide.